Infectious laryngotracheitis virus is a herpesvirus that causes a respiratory illness of varying virulence in chickens. Live attenuated ILTV vaccines are available to protect against the disease, but several reports have implicated vaccine viruses in the possible recurrence and spread of the disease (65 and 72), limiting vaccination to use in uninfected birds early in an outbreak. In order to design a more efficacious, attenuated vaccine, the genomic organization of the ILTV virus has been studied.
The ability to isolate viral DNA and clone this isolated DNA into bacterial plasmids has greatly expanded the approaches available to make viral vaccines. The methods used to make the present invention involve modifying cloned viral DNA sequences by insertions, deletions and single or multiple base changes. The modified DNA is then reinserted into the viral genome to render the virus non-pathogenic. The resulting live virus may then be used in a vaccine to elicit an immune response in a host animal and to protect the animal against a disease.
One group of animal viruses, the herpesviruses or Herpetoviridae, is an example of a class of viruses amenable to this approach. These viruses contain 100,000 to 200,000 base pairs of DNA as their genetic material. Importantly, several regions of the genome have been identified that are nonessential for the replication of virus in vitro in cell culture. Modifications in these regions of the DNA may lower the pathogenicity of the virus, i.e., attenuate the virus. For example, inactivation of the thymidine kinase gene renders human herpes simplex virus non-pathogenic (1), and pseudorabies virus of swine non-pathogenic (2).
Removal of part of the repeat region renders human herpes simplex virus non-pathogenic (3, 4). A repeat region has been identified in Marek's disease virus that is associated with viral oncogenicity (5). A region in herpesvirus saimiri has similarly been correlated with oncogenicity (6). Removal of part of the repeat region renders pseudorabies virus non-pathogenic (U.S. Pat. No. 4,877,737, issued Oct. 31, 1989). A region in pseudorabies virus has been shown to be deleted in naturally-occurring vaccine strains (7, 8) and it has been shown that these deletions are at least partly responsible for the lack of pathogenicity of these strains.
It is generally agreed that herpesviruses contain non-essential regions of DNA in various parts of the genome. Some of these regions are associated with virulence of the virus, and modification of them leads to a less-pathogenic virus, from which a vaccine may be derived.
Infectious laryngotracheitis virus (ILTV), an alpha herpesvirus (9), is an important pathogen of poultry in the USA, Europe, and Australia, responsible for egg production losses and death (10). It causes an acute disease of chickens which is characterized by respiratory depression, gasping and expectoration of bloody exudate. Viral replication is limited to cells of the respiratory tract wherein infection of the trachea gives rise to tissue erosion and hemorrhage.
In chickens, no drug has been effective in reducing the degree of lesion formation or in decreasing clinical signs. Vaccination of birds with various modified forms of the ILT virus derived by cell passage and/or tedious regimes of administration have been used to confer acceptable protection in susceptible chickens. Due to the limited degree of attenuation of current ILTV vaccines care must be taken to assure that the correct level of virus is maintained: enough to provide protection, but not enough to cause disease in the flock (11–21). Furthermore, these viruses may revert back to virulence, causing disease rather than providing protection against it.
ILTV has been analyzed at the molecular level. Restriction maps of the ILTV genome have been reported (22–26). The DNA sequence of several genes have been identified, i.e., thymidine kinase (27, 28), glycoprotein gB (27, 29, 30), ribonucleotide reductase (27, 31), capsid p40 (31, 32).
Furthermore, Shepard, et al. (53) disclosed that several genes located in the unique long region of the infectious laryngotracheitis virus genomic DNA are non-essential for viral replication.
Applicants have unexpectedly found that the unique short region of the ILT virus genomic DNA contains genes that are associated with ILTV virulence and that a deletion in those genes leads to an attenuated ILTV. Particularly, it was found that a deletion in the glycoprotein G (gG) gene of the ILT virus results in an attenuated virus, which is useful as a vaccine against subsequent attack by a virulent ILTV strains.
Applicants also found that a deletion in the glycoprotein I (gI) gene of the unique short region also attenuates the ILTV. Furthermore, it is contemplated that a deletion in the US2 gene, the UL-47 like gene, and the glycoprotein g60 gene of the unique short region will also attenuate the ILTV.
ILTV can become latent in healthy animals which makes them potential carriers of the virus. For this reason, it is clearly advantageous to be able to distinguish animals vaccinated with non-virulent virus from animals infected with disease-causing wild-type or naturally-occurring virus. The development of differential vaccines and companion diagnostic tests has proven valuable in the management of pseudorabies disease (55). A similar differential marker vaccine should be of great value in the management of ILTV caused disease. The construction of differential diagnostics has focused on the deletion of glycoproteins. Theoretically, the glycoprotein chosen to be the diagnostic marker should have the following characteristics: (1) the glycoprotein and its gene should be non-essential for the production of infectious virus in tissue culture; (2) the glycoprotein should elicit a major serological response in the animal; and (3) the glycoprotein should not be one that makes a significant contribution to the protective immunity.
The ILT virus has been shown to specify at least four major glycoproteins as identified by monoclonal antibodies (Mr=205K, 115K, 90K and 60K). Three glycoproteins seem to be antigenically related (Mr=205K, 115K, and 90K) (34–36).
Three major ILT virus glycoproteins, gB (29, 30), gC (27, 51), and g60 (34, 53) have been described in the literature. These three genes have been sequenced and two of the ILTV genes have been shown to be homologous to the HSV glycoproteins gB, and gC.
Of these, it is known that the ILTV gB gene is an essential gene and would not be appropriate as deletion marker genes. Furthermore, the gC gene of herpesviruses has been shown to make a significant contribution to protective immunity as a target of neutralizing antibody (56) and as a target of cell-mediated immunity (57). Therefore, the gC gene is not desirable as a deletion marker gene.
As to other glycoprotein encoding genes cited above, it is not known whether or not they would be suitable candidates for deletion in order to construct a recombinant ILT virus which can be used as a diagnostic vaccine.
Applicants have unexpectedly found that there are two glycoprotein encoding genes located within the unique short region of the ILT viral genome which could be safely deleted in order to construct a recombinant ILT virus that can be used as a diagnostic vaccine. These are the glycoprotein gG gene and the glycoprotein
gI gene. By genetically engineering an ILT virus with a deletion in the glycoprotein G gene or the glycoprotein I gene, a ILT virus is produced which does not express any glycoprotein G or glycoprotein I. None of the prior arts teach or suggest that these two genes in the unique short region of the virus are appropriate candidates for deletion in order to create a diagnostic ILT virus vaccine. Although several of the herpesviruses have been genetically engineered, no examples of recombinant ILTV have been reported.
The ability to engineer DNA viruses with large genomes, such as vaccinia virus and the herpesviruses, has led to the finding that these recombinant viruses can be used as vectors to deliver vaccine antigens and therapeutic agents for animals. The herpesviruses are attractive candidates for development as vectors because their host range is primarily limited to a single target species (37) and they have the capacity for establishing latent infection (38) that could provide for stable in vivo expression of a foreign gene. Although several herpesvirus species have been engineered to express foreign gene products, recombinant infectious laryngotracheitis viruses expressing foreign gene products have not been constructed. The infectious laryngotracheitis viruses described above may be used as vectors for the delivery of vaccine antigens from microorganisms causing important poultry diseases. Other viral antigens which may be included in a multivalent vaccine with an ILTV vector include infectious bronchitis virus (IBV), Newcastle disease virus (NDV), infectious bursal disease virus (IBDV), and Marek's disease virus (MDV). Such multivalent recombinant viruses would protect against ILT disease as well as other diseases. Similarly the infectious laryngotracheitis viruses may be used as vectors for the delivery of therapeutic agents. The therapeutic agent that is delivered by a viral vector of the present invention must be a biological molecule that is a by-product of ILTV replication. This limits the therapeutic agent in the first analysis to either DNA. RNA or protein. There are examples of therapeutic agents from each of these classes of compounds in the form of anti-sense DNA, anti-sense RNA (39). ribozymes (40), suppressor tRNAs (41), interferon-inducing double stranded RNA and numerous examples of protein therapeutics, from hormones, e.g., insulin, to lymphokines, e.g., interferons and interleukins, to natural opiates. The discovery of these therapeutic agents and the elucidation of their structure and function does not necessarily allow one to use them in a viral vector delivery system, however, because of the experimentation necessary to determine whether an appropriate insertion site exists.
ILTV is classified as an alpha herpesvirus with a type D genome (78) composed of a unique long region and a unique short region flanked by inverted repeats. A genomic restriction map of an Australian ILTV isolate (SA-2) was described by Johnson et al. (66). Using this map, Guo et al. (62) isolated and sequenced a DNA fragment from the USDA challenge strain which appeared to be derived from the unique short region. Applicants map the USDA challenge strain of ILTV, and reports characteristics of the putative genes present in the unique short region. The map disclosed herewith indicates that the sequence identified by Guo et al. (62) is part of the short repeat sequence, and is not from the unique short. Other reports (69 and 70) describe the sequences of two genes, one homologous to PRV gG and the other unlike other reported herpesvirus genes. These two genes were mapped to the unique long region of SA-2. However, these sequences are identical to sequences identified in this application as being from the unique short region. The data in this application indicate that the overall organization of the short region of ILTV is similar to other herpesviruses.